For example, Japanese Laid-Open Patent Publication No. 2007-238129 discloses a plastic bag used as an internal bag received in a bag in box. In the internal bag, a composite film is composed of external and internal films opposed to and superposed with each other, to have opposite end portions. The internal bag includes two of the composite films opposed to and superposed with each other. A process is then performed to join all the films with each other at the opposite end portions so that first joining portions should be formed between the external and internal films, a second joining portion being formed between the internal films, to make the plastic bag which is used as the internal bag. The external and internal films comprise plastic films.
In this case, the external and internal films are subjected to an impact load when dropping the internal bag so that a stress must be concentrated on the second joining portion formed between the internal films. The internal films may therefore be peeled and separated off from each other at the second joining portion, resulting in break of the internal bag.
The same is true of the internal bag received in the bag in box of Japanese Laid-Open Patent Publication No. 2011-235933 and Japanese Patent Publication No. 3324953.
By the way, it is usual when joining all the films with each other that a heater is pressed against the external and internal films to heat seal and join all the films with each other. In addition, it is practicable to irradiate laser-beam between and onto the external and internal films, welding and joining all the films with each other. See Japanese Laid-Open Patent Publication No. 2009-255461, Japanese Patent Publication No. 4404288. Japanese Laid-Open Patent Publication No. S60-157842 and Japanese Patent Publication No. 4104073.
A handle may be formed of the composite film composed of the external and internal films.
It is therefore an object of the invention to provide a plastic film joining method, which can keep the plastic films from being peeled off from each other at a concerned joining portion.